What does the fraction math node do?
Fract returns the fractional part of a floating-point value, as described by @Merlin, but its treatment of negative fractional parts is not actually the same as Blender's implementaion of Modulo.
If you want to see what a mathematical chunk of your shader tree is doing, it can be quite handy to make it generate a graph.
Fract:

This plane is 8x8 units, with its origin, (so (0,0)), at its center. I've added the annotations, but you can see the function without them.
Modulo 1:

Thanks to @Rich Sedman for the very useful graphing tip.